This invention relates to an optical amplifying apparatus which is operable as serves as an optical repeater amplifier used in an optical wavelength division multiplexing (will be abbreviated to WDM) communication system.
Heretofore, it is known in the art that a WDM communication system is capable of fast communicating information in a large capacity. This is because such a WDM communication system enables simultaneous communication of optical individual or component signals which have wavelengths different from one another on the same optical fiber. Herein, it is to be noted that the optical component signals are transmitted in the form of an optical wavelength multiplexed signal through on the same optical fiber that is operable as an optical transmission path and that the optical multiplexed signal has a transmission spectrum divided into a plurality of wavelength bands which are not overlapped with one another.
In such a WDM communication system, an optical amplifying apparatus is usually placed on the optical transmission path between an optical transmitter and an optical receiver to provide long distance communication.
In long distance optical communication systems using the optical amplifying apparatus, an output level of the optical amplifying apparatus should be kept at the best value to provide optimum transmission performance, in consideration of a non-linear effect of an optical fiber and optical noise (ASE: amplified spontaneous emission noise) in the optical amplifying apparatus.
This shows that an amplification gain must be favorably controlled in the optical amplifying apparatus to obtain the optical wavelength multiplexed signal with an optimum and constant output level. To this end, the conventional long distance optical communication systems monitor an output signal which is produced from the optical amplifying apparatus in the form of the optical wavelength multiplexed signal. The optical amplifying apparatus is controlled by the monitored result so that the optical wavelength multiplexed signal is kept at the optimum output level.
Under the circumstances, it should be noted that the individual or optical component signals which are multiplexed into the optical wavelength multiplexed signal and which are transmitted through the same optical fiber may be reduced in number due to a trouble or fault in the optical transmitter or on an upstream side. On the contrary, the optical individual signals may be increased in number when the number of the optical transmitters and the optical receivers is increased so as to cope with a larger transmission capacity.
Even if the number of the optical component signals is varied as mentioned above, it often happens that the optical wavelength multiplexed signal amplified by the optical amplifying apparatus is kept substantially invariable in an output level because the amplification gain of the optical amplifying apparatus is controlled with reference to the optical wavelength multiplexed signal as described above.
Herein, it is considered that the number of the optical component signals is reduced or increased. In this event, each level of the optical component signals is increased or reduced excessively. The reason for this is that the optical amplifying apparatus detects as the output level thereof only the optical wavelength multiplexed signal without detecting each level of the optical individual signals and controls the amplification gain on the basis of the optical wavelength multiplexed signal detected.
Such an individual excessive variation of the optical component signals often brings about a serious problem. For example, the optical receiver in the WDM communication system can normally receive the optical wavelength multiplexed signal but the optical receiver can not normally and individually receive each of the optical component signals. As the result, each necessary and original information is not accurately and individually monitored from the wavelength multiplexed signal. Namely, the WDM communication system can neither carry out nor guarantee accurate communication function thereof essentially.